I think most will say Star Trek 2: The Wrath of Kahn. I can't narrow it down like that, I don't have one favorite. My favorites include Wrath of Kahn, The Voyage Home, The Undiscovered country, Generations and First Contact.

The Wrath Of Khan **
Voyage Home
The Undiscovered Country **
First Contact

** these two, both by director Nicholas Meyer are probably the best Star Trek films ever.

---------------------------------------------------My avatar will Wait Until Dark for a Roman Holiday since this world as War & Peace
and it would be fun to actually have Breakfast At Tiffany's with My Fair Lady